The invention relates to a cathode-ray tube having an electron gun assembly therein and more particularly to the cathode assemblies of such a gun assembly wherein emissivity modifying means are included in less than all of the cathode assemblies.
A typical cathode-ray tube, e.g., a color television picture tube, includes an electron gun assembly comprising three substantially identical electron guns, each of which generates and focuses an electron beam which impinges on a different color phosphor element disposed on an interior surface of the tube faceplate. In a conventional tube, the phosphor elements are red-, green-, and blue-emitting stripes or dots, which are stimulated to emission by the aforementioned electron gun assembly. The electron gun that provides an electron beam which impinges on the red-emitting phosphor element will be hereinafter referred to as the red gun. The same convention will also apply to the green gun and the blue gun.
In a television receiver, it is desirable that when the receiver is switched on, the picture appears quickly and with the proper colors. This capability is directly related to the rate of increase in electron gun cathode temperature. Some receiver manufacturers require that the initial screen color (with no video signal) be either white or green, rather than red, blue or purple. A white screen indicates equal color balance between the red, green and blue guns. A green color indicates that the cathode of the green gun has reached emission temperature before the cathodes of the red and blue guns.
In the prior art, it was known to preheat the cathode heater to meet the demand of quick turn-on. However, even a quick turn-on system does not always provide a white or green initial screen color. Furthermore, the preheated cathodes use power when the television receiver is out of use. Such a structure is energy inefficient.
U.S. Pat. No. 4,184,100, issued to Takanashi et al. on Jan. 15, 1980, discloses an indirectly-heated cathode device of the quick-heat type. A cathode sleeve in the Takanashi et al. patent is formed from one of a number of high strength alloys, which permits the fabrication of a thin wall cathode sleeve, which is free from deformations and has small heat capacity. The Takanashi et al. patent addresses the quick-heat problem but does not suggest a structure for providing an initial white or green screen color.
U.S. Pat. No. 4,370,588, issued to Takahashi et al. on Jan. 25, 1983, discloses a low power, quick turn-on cathode. In the Takahashi et al. patent, a cathode sleeve is blackened, and a first reflective cylindrical member is fixed on the top end of the cathode sleeve. A second reflective cylinder, having a larger diameter than the cathode sleeve, is attached to the sleeve by support members. The reflective cylinders reflect radiated heat back to the cathode sleeve to maintain the cathode temperature and shorten the turn-on time. Such a structure does not specifically address the problem of having a white or green screen color when the cathode reaches emission temperature.
As disclosed in U.S. Pat. No. 4,071,803, issued to Takanashi et al. on Jan. 31, 1978, it is difficult to control the variation of the cathode electrode-to-grid electrode spacing due to temperature variation. "White balance", i.e., white screen color, is affected by dimensional variations in the cathode-to-grid electrode spacing so that, even with quick turn-on cathodes of substantially identical construction, "white balance"may not be immediately achieved, and the picture does not become normal until the dimension of the electron gun structure becomes thermally stable.
Since an initial white screen color requires that all three electron guns reach emission temperature substantially simultaneously, it is more convenient to provide an initial green screen color than it is to achieve a white screen color.